Agaricus deardorffensis — California flat-top agaricus

Odour: Strongly like phenol or creosote.
Cap: 5–15 cm in diameter, broadly convex, becoming flat with age. The cap is covered by dark grey-brown to dark blackish brown radial streaks and tufts on a light background. The centre is darkest, and the cap margin is pale. When scratched, the margin turns bright yellow, which fades after a few minutes.
Gills: Free, crowded, pink at first, dark brown with age.
Stems: 11–15 cm long x 1.5–2 cm wide, smooth, cylindrical and sometimes wider at base. The colour is white, but changes to yellow when scratched. The stem is hollow.
Ring: White, rubbery, hanging, sometimes becoming loose from the stem.
Cup: None.
Spores: 5.0–5.5 x 3.5–4 µm, smooth and dark brown.
Habitat: On the ground, often along roads and paths, in open forests and in urban settings, in various mixed forests.
Geographic distribution: Widespread in western North America, from BC (Canada) in the north into California (coastal and montane). Newly described in 2016, this species is likely common although it is at present known from few BC and Pacific northwest collections.

The California flat-top agaricus is one of several similar-looking, poisonous species in Agaricus sect. Xanthodermatei with a characteristic phenolic odour (also described as “library paste”) and a yellow discolouration upon bruising that fades. Other species in Xanthodermatei that share the dark fibrillose cap include Agaricus buckmacadooi, also found in the Pacific northwest and in coastal California, and Agaricus berryessae from Solano Co. in California. Morphological characters to distinguish among these poisonous species are subtle. Further complicating naming, in older literature these poisonous species were listed as Agaricus moelleri, A. meleagris or A. praeclaresquamosus. All three names refer to the European A. moelleri. Agaricus placomyces has also been used but is an eastern North American species.

Their smooth stems and phenolic odours distinguish poisonous members of Xanthodermatei from the edible Agaricus augustus (the prince), which is similar in size and habitat but which has a shaggy stem and almond odour.

Symptoms: All species in Agaricus sect. Xanthodermatei cause gastrointestinal distress, which can be severe. Symptoms include nausea and vomiting, which can begin almost immediately or up to ~8 hours after consumption. Symptoms usually resolve themselves, leading to full recovery.

Treatment: Contact your regional Poison Control Centre if you or someone you know is ill after eating California flat-top agaricus. Poison centres provide free, expert medical advice 24 hours a day, seven days a week. If possible, save the mushrooms or some of the leftover food containing the mushrooms to help confirm identification.

Poison Control:
British Columbia: 604-682-5050 or 1-800-567-8911.
United States (WA, OR, ID): 1-800-222-1222.
Symptoms usually resolve themselves in 24 hours.

Cases of poisoning: California flat-top agaricus specimens5,6 that were identified by DNA sequencing have been implicated in two cases of poisoning reported to the BC Centre for Disease Control Drug and Poison Information Centre.
  • In one incident, a man cooked and ate mushrooms6 in fried rice and experienced diarrhoea ~7 hours later followed by a headache and nausea. He went to the hospital 18 hours after eating the mushroom, with vomiting and with continuing diarrhoea.
  • The other patient had similar symptoms, and both patients recovered fully.

The California flat-top agaricus may turn out to be one of the more common causes of poisoning by mushrooms in BC and the Pacific northwest. Records are few because the species was so recently described and earlier poisonings would have been blamed on look-alikes such as A. praeclaresquamosus.

MyCoPortal. Mycology Collections Portal, <> accessed February 2018.

Specimen Agaricus deardorffensis UBC F32580, GenBank #MH718225.

Kerrigan, R. W. Agaricus of North America. Memoirs of The New York Botanical Garden. Vol. 114, NYBG Pres, Bronx, New York (2016).

Siegel, N. & Schwarz, C. Mushrooms of the Redwood Coast. A Comprehensive Guide to the Fungi of Coastal Northern California. Ten Speed Press, Berkeley, California (2016).

Specimen Agaricus deardorffensis UBC F32558, GenBank #MG969965.

Specimen Agaricus deardorffensis UBC F32578, GenBank #MH718224.